Source : Webster's Revised Unabridged Dictionary (1913)

Red-letter \Red"-let`ter\ (-l?t`t?r), a.
   Of or pertaining to a red letter; marked by red letters.

   {Red-letter day}, a day that is fortunate or auspicious; --
      so called in allusion to the custom of marking holy days,
      or saints' days, in the old calendars with red letters.